Symonds, Rose, Bluth, and Gerlach concluded that stratospheric injection of sulfur dioxide (SO2) is the principal atmospheric and global impact of volcanic eruptions via
SO2 + OH + 3H2O -> H2SO4 (l) + HO2
The amount of SO2 released by volcanoes is much less compared to man-made sources but the impact of some eruptions might be disproportionately large. The gases emitted by most eruptions and by man-made sources never leave the troposphere, the layer in the atmosphere from the surface to about 10 km.
However, volcanic gases reach the stratosphere, a layer in the atmosphere from about 10 km to about 50 km in altitude, during large eruptions. This relationship is complicated by the fact that the elevation between the volcano summit and the distance to the troposphere/stratosphere decreases with latitude. So, some smaller eruptions at higher latitudes can eject as much SO2 gas into the stratosphere as larger eruptions closer to the equator.
Factors influencing the amounts of SO2 in the stratosphere were described and modeled by Bluth and others (1997). For eruptions in the last 25 years, El Chichon and Mount Pinatubo emitted the greatest amounts of SO2 into the stratosphere. El Chichon produced 7 Mt of SO2 and Mount Pinatubo produced 20 Mt. Both of these volcanoes are at low latitudes but they both had high eruption rates.
The importance of latitude is obvious for four of the next five volcanoes that had a major influence on SO2 amounts in the stratosphere. Hudson, St. Helens, Alaid, and Redoubt are all at latitudes greater than 45 degrees, where the distance to the stratosphere is less. The eruption rate of Hudson was comparable to El Chichon and Mount Pinatubo. However, the eruption rates of St. Helens, Alaid, and Redoubt where an order of magnitude less. These volcanoes emitted 1, 1.1, and 0.2 Mt of SO2. The other eruption was at Ruiz, which had a high eruption rate, comparable to El Chichon and Mount Pinatubo, but is near the equator. Ruiz emitted 0.7 Mt of SO2. Bluth and others (1997) used the changes in aerosol optical depth as a measure of the impact of the eruptions.
The impact of eruptions may not last very long. The aerosols in the stratosphere from mid-range eruptions (St. Helens, Alaid) settled back to the troposphere in about 5-8 months (Kent and McCormick, 1984). For large eruptions like El Chichon it takes about 12 months for SO2 levels in the stratosphere to return to pre-eruption levels. Pinto and others (1989) suggested that at high eruption rates aerosols tend to make larger particles, not greater numbers of same size aerosol particles. Larger particles have smaller optical depth per unit mass, relative to smaller particles, and settle out of the stratosphere faster. These self-limiting effects may restrict the total number of particles in the stratosphere and may moderate the impact of volcanic clouds (Rampino and Self, 1982; Pinto and others,1989).
More complicated patterns of warming and cooling have been found on regional scales. Robock and Mao (1992) found warming over Eurasia and North America and cooling over the Middle East and northern Africa during the winters after the 12 largest volcanic eruptions from 1883-1992. For eruptions in the tropics the temperature changes were noted in the first winter after the eruption. For eruptions in the mid-latitudes changes were observed in the first or second winter after the eruption. For eruptions in high latitudes changes were observed in the second winter after the eruption. Robock and Mao (1992) proposed that heating of the tropical stratosphere by the volcanic aerosols led to an enhanced zonal winds. The zonal winds heated some areas while blocking of solar radiation cooled other areas.